In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Dry plates, which utilize an oleophobic topmost layer of fluoropolymer or, more commonly, silicone (polydiorganosiloxane), exhibit excellent debris-trapping properties because the topmost layer is tough and rubbery; ablation debris generated thereunder remains confined as the silicone or fluoropolymer does not itself ablate. Where imaged, the underlying layer is destroyed or de-anchored from the topmost layer. A common three-layer plate, for example, is made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, leaving the topmost layer de-anchored in the exposed areas. Subsequently, the de-anchored overlying layer and the central layer are removed (at least partially) by a post-imaging cleaning process—e.g., rubbing of the plate with or without a cleaning liquid—to reveal the third layer (typically an oleophilic polymer, such as polyester).
To be viable commercially, printing members must be able to withstand a variety of predictable environments for relatively long periods of time. Lithography is carried out on a worldwide basis in installations ranging from high-volume industrial operations to small print shops. Although traditional photosensitive plates naturally exhibited limited shelf-life as a consequence of radiation sensitivity, even ablation-type plates can degrade over time. Although they require high exposure fluences to remove an energy-absorbing layer in order to create an image, and therefore are not particularly sensitive to environmental radiation, multi-layer polymeric structures nonetheless remain vulnerable to other environmental conditions—temperature extremes, high relative humidity, and long exposure to—i.e., aging in—these environments. Most notably, aging substantially reduces the useful length of run on press. Whereas a new plate may achieve 20,000 impressions, an age-degraded plate under the same conditions will fail very early, e.g., after 1000 impressions. In a dry plate, the silicone layer falls away on-press and ink is accepted in unwanted regions of the plate. Age-degraded plates also have a tendency to scratch easily (e.g., due to the breakdown of silicone and/or its loss of adhesion to the underlayer).
Accordingly, there is a persistent need for improvements in plate shelf-life, i.e., long-term tolerance to stressful environmental conditions.